This work examined the conditions for synthesis as well as basic material properties of the high pressure modification of Sialon (spinel-Sialon). Phase analysis of the products of high-pressure and high-temperature syntheses resulted in better understanding of the conditions needed for a successful beta to gamma transformation. Rietveld-analysis of diffractograms taken at different temperatures yielded in the temperature-dependent coefficient of thermal expansion for the composition x = 2,1 in the temperature range 20-1220°C. Combination of Vickers and Knoop hardness measurements along with a measurement of the compressibility of a particular composition (x = 2,1) have been used to determine some mechanical properties, mostly in dependency of the composition. Among the properties were load-independent hardness, Young’s modulus, bulk modulus, shear modulus and Poisson ratio (the latter three for the composition, which has been used for the compressibility mesurements). A combination of NEXAFS- und SXFS-measurements revealed the electronic structure near the Fermi level for different compositions of beta- and gamma-Sialons. Those measurements were used to approximate the electronic bandgap of those compositions.